The present invention concerns a process for the underwater plasma cutting of workpieces, especially structural steel.
The state of the art is exemplified by U.S. Pat. No. 3,851,864. According to this prior art cutting method, the cutting process is carried out with a plasma burner at a small distance under a liquid level. By carrying out the cutting process in water, the entire cutting process generates less environmental nuisance in terms of noise, dust, gases, etc. The type of plasma burner used is, however, not specified in U.S. Pat. No. 3,851,864.
Another process for avoiding environmentally hazardous components during the plasma cutting or workpieces by means of a plasma burner is also known from German Offenlegungsschrift No. 25 05 665, in which the workpiece and the plasma burner art also submerged in a liquid bath for cutting, so that the cutting process is carried out at a small distance under the liquid level. It is proposed in the German Offlegungsschrift that a plasma cutting burner with a liquid-stabilized, transmitted plasma cutting beam be used, with the plasma cutting gas consisting mainly of nitrogen. Such a burner, as it has become known, for example, from U.S. Pat. No. 3,641,308, has the advantage that as a consequence of the use of nitrogen as a plasma cutting gas, it is possible to cut structural steels burr-free. Due to the flat electrode used with this burner, however, it is necessary to supply a large electrical power, so that the power factor (ratio of continuous power consumption to cutting power) is obtained that is more unfavorable than the power factor than can be reached with a plasma burner having a needle electrode. Plasma cutting burners of this kind with needle electrodes are known from German Pat. No. 20 19 946. These burners are, as noted above, favorable with respect to the power factor, but they operate with mixtures of argon, hydrogen and nitrogen, which is unfavorable for achieving optimally burr-free cuts in structural steel.
There has been a steady desire in industry to reduce the amount of electrical requirements for underwater plasma cutting as well as in other areas, while maintaining the cutting quality already achieved. In view of the above-described state of the art, it would seem logical for the expert that in order to reduce the energy, a plasma burner with a needle electrode and with nozzles strongly contracting the arc, i.e., a plasma burner corresponding to German Pat. No. 20 19 946, should be used also fur underwater plasma cutting. Practice has, however, shown that during the ignition of the plasma cutting arc, and also during plasma cutting proper, uncontrollable and unintended arcs are generated between the contracting nozzle and the workpiece, and a so-called double arc is formed. These double arcs (the arc between the electrode and the nozzle, and the arc between the nozzle and the workpiece) lead to the sudden destruction of the nozzles, so that their life becomes so short that the desired mode of operation (underwater plasma cutting with a plasma burner with needle electrode) is no longer economical. It has also been shown that the burr-free cutting of structural steel under water is also not achieved with these burners and the plasma gases (argon, hydrogen and nitrogen mixture).